As is known in the prior art, in the drying section of a paper machine, single-wire draw and/or twin-wire draw is/are applied.
Single-wire draw, in which the drying wire also supports the web on the draws between the rows of cylinders, is usually employed in the initial part of the drying section. Single-wire draw may also be used over the entire length of the drying section.
Recently, drying sections provided with single-wire draw have become common in which the upper or lower cylinders are steam-heated drying cylinders against which the web is placed in direct contact pressed by the drying wire, and in which the lower or upper cylinders are cylinders provided with internal suction, for example the assignee's so-called "VAC-ROLL.TM." cylinders. In "VAC-ROLL.TM." cylinders, the effect of negative pressure is applied through the perforated mantle from the interior of the leading cylinder to the grooves passing around the cylinder mantle. By means of the effect of negative pressure, the web is kept in contact with the drying wire when the web is placed at the side of the outside curve on the leading cylinders. At the same time, attempts are made to prevent transverse shrinkage of the web as the drying makes progress.
Typically, in a multi-cylinder dryer, there are 5 to 8 wire groups. The groups placed in the initial end of the drying section are normally shorter than the groups placed in the final end.
With regard to prior art related to the present invention, reference is made to FI Pat. Appl. 793,920; FI Published Patent Application 70,277; and to DE Patent 1,183,775. In these publications, various devices for profiling of the moisture content in a paper web are described. These devices are, however, not employed for the control of anisotropy in the direction of thickness of paper nor for the control of the tendency of curling.
As is well-known, the lateral portions of a paper web become dryer than the middle portion of the web in the drying section of a paper machine. This flaw of moisture profile is commonly corrected after said principal drying stage either by drying the middle area additionally by means of infrared radiators divided in zones or by moistening the edges by means of water sprayed in zones. Both of these prior-art modes increase the relative slackness of the edges in relation to the middle portion.
The tension profiles of paper in the transverse direction and in the thickness direction depend on the moisture profiles of the paper. On the other hand, the tension of paper on drying has an effect on the properties of the paper. When the moisture profile of paper is affected, e.g., during drying, the properties of the paper are also altered at the same time. The properties of paper, for example tensile strength, are developed extensively at a dry solids content of about 75 to about 98%. Thus, the tension profiles, expressly at the final stage of the drying, have a great importance in view of the ultimate properties of the paper.
When a paper machine dries the web unevenly in its transverse direction, this causes, among other things, uneven tension in the web. An uneven tension profile means, for example, that the edge of the paper web produced by the paper machine has more slack than the middle portion of the web. This is a common occurrence in presently available paper machines. By means of measurements, it has been found that, in the transverse profile of tension, peaks and valleys, i.e. tighter and slacker zones, also occur in the middle portions of the web.
In the stages of web treatment after the paper machines, unevenness of the tension profile in the web may produce remarkable handling and running problems, e.g., in the control of the structure of the reel for the customer, in formation of wrinkles, as breaks, and as problems of alignment in a printing machine.
The slack edge of a paper web can be explained by means of three well-known factors. First, in a conventional cylinder drying group, the edges of the web dry more quickly than the middle of the web. Second, the water-swollen fibers and the paper web shrink when the drying makes progress. This shrinkage is particularly intensive in the range of dry solids content of about 65 to about 95%. Third, the deformation of paper when moist is primarily plastic, whereas the force-elongation behavior of a paper of higher dryness is largely elastic. Thus, a deformation produced in moist paper, such as stretching, mostly remains permanent, whereas a stretching in a paper of higher dryness is largely restored, being lost when the force is gone.
Curling of paper is one of its negative quality factors. Reducing the tendency of curling of paper, in particular of fine paper, has become more and more important along with new printing and copying methods, in which the paper is heated suddenly, as a rule from one side, so that any internal strains that may have remained in the paper because of different reasons are discharged in the form its curling. The tendency of curling of paper is affected both by the anisotropies in the different properties of the structure of the paper, such as fiber orientation, filler distribution, and density, and by anisotropies produced in the paper during drying, both in the direction of the plane of the paper and in the z-direction (i.e., the direction of thickness).
It is a drawback of the drying groups with single-wire draw described above that they do not dry the paper symmetrically, but the drying effect is applied more extensively to the face of the paper that reaches direct contact with the heated drying cylinders. Owing to this asymmetric drying, recently such drying groups with single-wire draw have been introduced in which certain groups, for example, every second group, are so-called inverted groups, in which the steam-heated drying cylinders are in the lower row and the leading cylinders are in the upper row. However, in the inverted groups, difficulties occur in the removal of broke, because the free sectors of the drying cylinders are not opened downwards, as they are in normal groups, but they form pockets that are closed at the bottom. In the inverted groups, problems are also caused by the ventilation of the spaces that remain in the gaps between the leading cylinders as well as by the differences in pressure in the narrowing wedge spaces placed in connection with the free faces of drying cylinders in relation to the environments of said wedge spaces.
One factor that has a substantial effect on the curling of paper is one-sided drying of the paper. In drying sections provided with the traditional twin-wire draw, the web can be dried at the same rate from both sides. Thereby, the uniformity of drying can be affected, and the tendency of curling of paper can be prevented, by regulating the steam pressures in the upper and lower cylinders.
By means of the new concept of a dryer with single-wire draw, said drying at the same rate and said possibility of regulation cannot be accomplished. Attempts are made to prevent unequal drying on either side of the paper by means of the inverted drying groups, in which the above problems of cleaning, however, deteriorate the running quality of the paper machine.
In connection with calendaring, it is known in the prior art to employ steam spraying applied to the paper. The effect of steam spraying is based on relaxing the strains in the paper fibers by the effect of heat and moisture. However, in the calendaring stage, it is no longer possible to sufficiently eliminate the tendencies of curling of paper in all cases in an efficient manner.